1. Technical Field
The present invention relates to an oscillation circuit using a microstrip line such as a voltage control oscillation circuit (hereinafter referred to as VCO). More particularly, it relates to the VCO having a resonant circuit of the microstrip line whose line width and line length are minimized, in which the microstrip line and an additional capacitance component are made as one piece circuit elements, and furthermore the oscillation frequency is adjusted by trimming a stub with the microstrip line connected with the stub, and a cascode connecting amplifier which lessens the oscillation frequency fluctuation due to load variations is used.
2. Technical Background
Conventionally, it has been known widely that a VCO used in mobile communications, for example, in a car telephone or a satellite communications system, comprises a microstrip line and a dielectric coaxial resonator as a resonator of a resonant circuit.
However, an oscillator using the microstrip line has, in general, a poor C/N ratio (C: carrier wave, N: noise) as compared with the oscillator using the dielectric coaxial resonator, so that (excluding a portion of consumer products in which more importance is attached to a cost), in practice, the dielectric coaxial resonator is widely used.
FIG. 7 is an electric circuit diagram of the VCO having the resonant circuit of a dielectric coaxial resonator.
The VCO is a Colpitts oscillation circuit consisting of a negative resistance circuit 6a having an active element and a resonant circuit 6b having a dielectric coaxial resonator 61.
The VCO has a terminal Vt which is a control voltage terminal, a terminal M which is a modulation terminal, a terminal B which is a power terminal and a terminal P which is an output terminal.
A transistor Q1 is collector grounded by the negative resistance circuit 6a having the active element.
The resonant circuit 6b has a dielectric coaxial resonator 61, a variable capacitor Cv whose capacity varies by control voltages, bypass capacitors C1, C2, a capacitor C3 for determining the variable range of the voltage control oscillation frequency and a clap capacitor 4 and so on.
The dielectric coaxial resonator 61 used in such a resonant circuit 6b, as shown in FIG. 8(a), has a through hole 72 extending to the bottom from the upper surface 71 of a dielectric block, on the side of which is an outer conductor 74 and on the inner surface of the through hole 72 is an inner conductor 75. On the opposite surface (not shown) of the upper surface 71, the outer conductor 74 and the inner conductor 75 are interconnected. These conductors 74 and 75 may be formed of silver or other suitable materials.
When the length of the dielectric coaxial resonator 61 is designated at l.sub.0, resonance takes place when the frequency is 1/4.lambda.g (.lambda.g: guide wave length along line), and at l.sub.0 &lt;1/4.lambda.g showing an inductivity which is utilized by the VCO as the inductance of the resonant circuit for oscillation.
The relationship between the wave length .lambda.g where the dielectric coaxial resonator resonates and the length l.sub.0 of the dielectric coaxial resonator 61 is an equivalent circuit of FIG. 8(b) and may be represented as, ##EQU1## where, c: velocity of light in free space
f.sub.0 : resonant frequency PA1 .epsilon..sub.r : relative dielectric constant PA1 n: arbitrary integer
Here, when the dielectric coaxial resonator 61 having the dielectric constant .epsilon..sub.r of 90 is used to set the resonant frequency f.sub.0 at 900 MHz, the length l.sub.0 becomes about 8.8 mm. Equivalent inductance L.sub.0 at this time is about 1.55 nH and the equivalent capacitance C.sub.0 is about 20.sub.p.sup.F.
However, since the size of dielectric coaxial resonator 61 practically results in an electronic component of about 1 cm square and since the dielectric coaxial resonator must be soldered precisely onto a predetermined oscillation circuit substrate (not shown), there is such a disadvantage of complicating the mounting process and restricting the minimization of the entire oscillation circuit. It is also considered to replace the dielectric coaxial resonator 61 (impedance Z.sub.0 =6.OMEGA., dielectric constant .epsilon..sub.r =90) of the resonant circuit 6b with a microstrip line 81 as shown in FIG. 9. The microstrip line 81 equivalent to the aforesaid dielectric coaxial resonator 61 corresponds to those formed on a dielectric substrate 82 (alumina substrate of dielectric constant .epsilon..sub.r =9.6) by a conductor having the line width W of 9 mm and the line length L of 28.7 mm.
That is, forming the microstrip line 81 equivalent to the aforesaid dielectric coaxial resonator 61 practically on the dielectric substrate 82, for example, by a thick film process, considerably hinders the practical size minimization of the oscillation circuit, since the line width W of the microstrip line 81 becomes very wide, for example 9 mm.
When the line width W is narrowed to minimize the size of the microstrip line 81, a conductor resistance increases and a Q value is lowered. This eventually results in a resonant circuit having a poor selectivity and, thus, thereby noises near the oscillation frequency which are difficult to restrain or reduce.
In general, when the line width W of the microstrip line 1 is narrowed, as shown by the broken line in FIG. 5, the resonance impedance is lowered as compared with the case shown in the unbroken line, resulting n a gentle graph.
When the line width W is eventually reduced to make the microstrip line compact in size, the reduction of the Q value and the increment of the conductor resistance are developed and practical minimization becomes impossible. Thus, a resonator having equivalent or better characteristics than a dielectric coaxial resonator was not accomplished.